Industrial production of methanol is performed by using a fossil fuel as a feedstock, and by allowing a synthesis gas (hereinafter, also referred to as a “synthesis raw material gas” or “raw material gas”) mainly comprising carbon monoxide, carbon dioxide and hydrogen, obtained by reforming the fossil fuel, to react on a catalyst. The involved reaction conditions are such that the pressure is 50 to 150 kg/cm2, the temperature is 160 to 300° C., and the catalyst used is a catalyst mainly comprising copper/zinc. The methanol synthesis reaction is represented by the following formulas (1) and (2).[Formula 1]CO+2H2→CH3OH  (1)CO2+3H2→CH3OH+H2O  (2)
Patent Document 1 has pointed out a problem that in the methanol synthesis at a low circulation ratio, the reactant partial pressure of the gas is sometimes high, and this causes excessive reaction and the occurrence of overheating of the catalyst bed. Accordingly, Patent Document 1 proposes, in order to solve this problem, that the supplied synthesis raw material gas is divided into two flows, one flow is mixed with the circulation unreacted gas and then introduced into a first synthesis stage, the other flow is mixed with the outlet gas of the first synthesis stage, and before the separation of the synthesized methanol, methanol is synthesized at an additional synthesis stage. The technique disclosed in Patent Document 1 is characterized in that the overheating of a catalyst layer is avoided by regulating the synthesized amount of methanol at the synthesis stage, and at the same time, the circulation ratio represented as the flow rate of the circulation unreacted gas based on the flow rate of the supplied raw material gas can be made as low as 1 to 3.
Patent Document 2 states that the methanol synthesis performed under a low pressure provides an advantage such that the load on a compressor is reduced, or the compressor is made completely unnecessary, but, on the other hand, has a drawback such that a large amount of catalyst is required, or the unreacted gas is required to be recycled at a high circulation ratio. The technique disclosed in Patent Document 2 is characterized in that in order to solve such a drawback, two synthesis reactors are serially installed, and the outlet gas from each of the synthesis reactors is condensed and separated to reduce the circulation ratio so as to be 4.0 or less. Specifically, Examples in Patent Document 2 show that the circulation ratio was altered from 6.0 to 3.5.
Patent Document 3 discloses that higher partial pressures of reactants in reactors can lead to excessive reaction and high temperatures. The document discloses that these high temperatures may lead to a higher deactivation rate for the catalyst. Therefore, Patent Document 3 proposes a technique characterizing in that a plurality of reactors is placed in a synthesis loop; a separator is placed downstream of each of the reactors; the reactant gas can be fed upstream of the reactors and the pressure is increased between the reactors as means for enabling large volumes of the desired product to be produced in an economical manner without a reduction in catalyst life expectancy. Patent Document 3 discloses that the above-described technique enables the production of the desired product to be achieved whilst reducing the circulation rate of gases and controlling the temperatures within the reactors such that an acceptable catalyst life can be achieved. The example of the document indicates that 23% or approximately 28% of the circulation rate of gases is reduced.
However, when the economic efficiency improvement based on scaling up the plant is pursued, there is generally used a technique to achieve a large scale by parallelizing the points at the bottlenecks. For example, in the process of methanol synthesis, the scale of the plant is sometimes limited by, for example, the production restriction imposed on reactors. In such a case, a plurality of reactors is arranged in parallel to achieve a large scale of the whole plant, as the case may be.
Non Patent Document 1 also discloses the considerable degradation of the catalytic activity caused by the water produced by accompanying the methanol synthesis.
Non Patent Document 2 discloses the course of the development of the methanol synthesis technology. More specifically, the development of the production process of the methanol synthesis technology has been advanced with a focus on the pursuance of the improvement of the energy efficiency and the improvement of the economic efficiency on the basis of the achievement of a large scale of plant. Additionally, according to the disclosure in Non Patent Document 2, as effects accompanying a drastic decrease of the circulating amount of the unreacted gas, the reduction of the electric power used and the cooling water amount used, and the size reduction of the piping in a synthesis loop and peripheral devices such as a circulator and heat exchangers are made possible.